Philips Semiconductors

Product specification

NE/SA604A

High performance low power FM IF system

1994 Dec 15

404

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NE 604A

430

430

SR00320

Figure 10. Crystal Input Filter with Ceramic Interstage Filter

forms a divider from the output of the limiters back to the inputs

(including RF input). If this feedback divider does not cause

attenuation greater than the gain of the forward path, then oscillation

or low level regeneration is likely. If regeneration occurs, two

symptoms may be present: (1)The RSSI output will be high with no

signal input (should nominally be 250mV or lower), and (2) the

demodulated output will demonstrate a threshold. Above a certain

input level, the limited signal will begin to dominate the regeneration,

and the demodulator will begin to operate in a “normal” manner.

There are three primary ways to deal with regeneration: (1)

Minimize the feedback by gain stage isolation, (2) lower the stage

input impedances, thus increasing the feedback attenuation factor,

and (3) reduce the gain. Gain reduction can effectively be

accomplished by adding attenuation between stages. This can also

lower the input impedance if well planned. Examples of

impedance/gain adjustment are shown in Figure 9. Reduced gain

will result in reduced limiting sensitivity.

A feature of the NE604A IF amplifiers, which is not specified, is low

phase shift. The NE604A is fabricated with a 10GHz process with

very small collector capacitance. It is advantageous in some

applications that the phase shift changes only a few degrees over a

wide range of signal input amplitudes.

Stability Considerations

The high gain and bandwidth of the NE604A in combination with its

very low currents permit circuit implementation with superior

performance. However, stability must be maintained and, to do that,

every possible feedback mechanism must be addressed. These

mechanisms are: 1) Supply lines and ground, 2) stray layout

inductances and capacitances, 3) radiated fields, and 4) phase shift.

As the system IF increases, so must the attention to fields and

strays. However, ground and supply loops cannot be overlooked,

especially at lower frequencies. Even at 455kHz, using the test

layout in Figure 3, instability will occur if the supply line is not

decoupled with two high quality RF capacitors, a 0.1

µF monolithic

electrolytic is not an adequate substitute. At 10.7MHz, a 1

µF

tantalum has proven acceptable with this layout. Every layout must

be evaluated on its own merit, but don’t underestimate the

importance of good supply bypass.

At 455kHz, if the layout of Figure 3 or one substantially similar is

used, it is possible to directly connect ceramic filters to the input and

between limiter stages with no special consideration. At frequencies

above 2MHz, some input impedance reduction is usually necessary.

Figure 9 demonstrates a practical means.

As illustrated in Figure 10, 430

Ω external resistors are applied in

parallel to the internal 1.6k

Ω load resistors, thus presenting

approximately 330

Ω to the filters. The input filter is a crystal type for

narrowband selectivity. The filter is terminated with a tank which

transforms to 330

Ω. The interstage filter is a ceramic type which

doesn’t contribute to system selectivity, but does suppress wideband

noise and stray signal pickup. In wideband 10.7MHz IFs the input

filter can also be ceramic, directly connected to Pin 16.

In some products it may be impractical to utilize shielding, but this

mechanism may be appropriate to 10.7MHz and 21.4MHz IF. One

of the benefits of low current is lower radiated field strength, but

lower does not mean non-existent. A spectrum analyzer with an

active probe will clearly show IF energy with the probe held in the

proximity of the second limiter output or quadrature coil. No specific

recommendations are provided, but mechanical shielding should be

considered if layout, bypass, and input impedance reduction do not

solve a stubborn instability.

The final stability consideration is phase shift. The phase shift of the

limiters is very low, but there is phase shift contribution from the

quadrature tank and the filters. Most filters demonstrate a large

phase shift across their passband (especially at the edges). If the

quadrature detector is tuned to the edge of the filter passband, the

combined filter and quadrature phase shift can aggravate stability.

This is not usually a problem, but should be kept in mind.

Quadrature Detector

Figure 7 shows an equivalent circuit of the NE604A quadrature

detector. It is a multiplier cell similar to a mixer stage. Instead of

mixing two different frequencies, it mixes two signals of common

frequency but different phase. Internal to the device, a constant

amplitude (limited) signal is differentially applied to the lower port of

the multiplier. The same signal is applied single-ended to an

external capacitor at Pin 9. There is a 90

plates of this capacitor, with the phase shifted signal applied to the

upper port of the multiplier at Pin 8. A quadrature tank (parallel L/C

network) permits frequency selective phase shifting at the IF

frequency. This quadrature tank must be returned to ground through

a DC blocking capacitor.

The loaded Q of the quadrature tank impacts three fundamental

aspects of the detector: Distortion, maximum modulated peak

deviation, and audio output amplitude. Typical quadrature curves

are illustrated in Figure 12. The phase angle translates to a shift in

the multiplier output voltage.